A “bass reflex” loudspeaker (also known as a ported, vented box or reflex port loudspeaker) is a type of loudspeaker with an enclosure that uses the sound from the rear side of the diaphragm to increase the efficiency of the loudspeaker at low frequencies as compared to a typical closed box loudspeaker. Such loudspeakers employ a reflex port, which generally consists of one or more ducts mounted in the front face (baffle) or rear face of the enclosure, leading from the air volume behind the driver to the external air.
This results in a Helmholtz resonance that combines with the loudspeaker output to give additional low frequency output. In the simplest terms the air in the enclosure behaves as an acoustic compliance which combines with the acoustic mass of the air in the duct to form an acoustic bandpass filter. The acoustic output from the rear of the driver passes through this filter and combines with the output from the front of the driver. For a particular low frequency driver the box volume and duct dimensions are typically chosen to give a response which has the characteristics of a fourth order high-pass filter.
Reflex systems are widely used since they provide better combination of efficiency and low frequency extension compared to closed box systems. They also have the benefit of reducing the diaphragm excursion at frequencies around the enclosure tuning frequency where the duct provides the main acoustic output. Though helpful with extending bass performance, bass reflex cabinets can have poor transient response compared to sealed enclosures at frequencies near the lower limit of performance. Proper adjustment of the cabinet and port size, and matching with driver characteristics are the typical approaches used to address this problem.